1. Field of the Invention
This invention relates to a signal processing device for performing mute processing on a sigma-delta modulated one-bit signal.
2. Description of the Related Art
In a number of cases with a digital audio source, such as, a compact disc (CD) or a digital audio tape (DAT), a perfectly silent portion partly exists between programs or during musical performance. In a state of playback pause, an audio signal is muted. In the silent portion, since a 16-bit digital audio signal is formed in the form of consecutive 0000h or consecutive low-level DC data, the data includes no noise data.
In a CD player or the like, a 16-bit digital audio signal is muted by using a signal processing device as shown in FIG. 1. That is, the time of a 0000h portion of a 16-bit digital audio signal supplied via an input terminal 51 is measured by a silence portion detecting circuit 52. If the measurement result is equal to or longer than a predetermined time period, the measured portion is regarded as "silence", and an analog audio signal supplied via a digital-to-analog (D/A) converter 53 and a filter and amplifier 54 is electrically muted by a muting circuit 55.
At the time of silence, noise generated by the D/A converter 53 and residual noise of a low-pass filter, a line amplifier and the like are not outputted. Therefore, the signal-to-noise ratio (hereinafter referred to as S/N ratio) may be improved.
Meanwhile, it has been recently considered to apply a one-bit audio signal obtained by performing sigma-delta (.SIGMA..DELTA.) modulation on an analog audio signal, to a recorder or data transmission of high sound quality. The one-bit audio signal obtained by .SIGMA..DELTA. modulation has a very high sampling frequency and a short data word length, specifically, a sampling frequency which is 64 times of 44.1 kHz and a data word length one bit, compared with a multi-bit digital signal having a sampling frequency of 44.1 kHz and a data word length of 16 bits which is used for conventional digital audio. Thus, the one-bit audio signal obtained by .SIGMA..DELTA. modulation is characterized by a wide transmissible frequency band. In addition, even though the signal is the one-bit signal obtained by .SIGMA..DELTA. modulation, a high dynamic range may be secured in a conventional audio band (20 kHz), which is a lower range with respect to an oversampling frequency of 64 times 44.1 kHz.
With the 16-bit digital audio signal, the consecutive 0000h portion may be regarded as a silence portion, as described above. However, in the case of the one-bit audio signal obtained by .SIGMA..DELTA. modulation, the only possible data is a positive maximum value or a negative maximum value without having a value equal to 0, and therefore the silence portion cannot be detected by the above-described technique.
In short, 1 of the one-bit audio signal represents a positive (+) maximum value while 0 represents a negative (-) maximum value. Thus, the intermediate value .+-.0 between these maximum values cannot be represented by any means. For example, consecutive one-bit audio signals 0 result in continuous output of the (-) maximum value. Therefore, when the one-bit audio signal is used, it is impossible to detect the silence portion and mute the output so as to improve the S/N ratio.
Thus, the present Applicant has proposed, in the U.S. patent Ser. No. 08/754270 (Filing date: Nov. 20, 1996), a technique for recording a one-bit mute signal onto a recording medium with respect to a recording device for the one-bit digital audio signal.
However, the above application discloses a method of recording a mute signal onto a recording medium but does not disclose any specific example relating to reproduction in which the one-bit digital mute signal recorded on the recording medium is extracted and muted.